IPHONE 6

The above is true, but on our Samsung phones, we can turn off data and still have operable phone. Do it quite often.

You must have a data plan to use any iPhone

To the best of my knowledge there if you turn off the mobile data service the phone will become inoperable. In other words you will not be able to make any phone calls. That is what happened when we attempted to do that with my DW's iPhone 4.